1.Field of the Invention
The invention relates in general to the maintenance of nuclear steam generator tubes, and more specifically to an orifice insert for placement into nuclear steam generator tubes for primary temperature reduction.
2. Description of the Prior Art
A nuclear steam generator 10 of the type found in the art is shown in FIG. 1 of the attached drawings as comprising an array of a large number of vertically oriented U-shaped tubes 12. The tubes 12 are disposed in a cylindrical portion 14 ot the steam generator 10 whose bottom end is associated with a radiation confining housing or channel head 16, typically having a bottom portion or bowl 18 of a hemispherical configuration. The channel head 16 is divided by a divider plate 20 into a first half 22 typically known as the hot leg plenum, and a second half 24 typically known as the cold leg plenum. An inlet pipe (not shown) supplies hot water to the hot leg plenum 22, while a return pipe 26 is coupled to the cold leg plenum 24. The hot water entering the hot leg plenum 22 passes into the exposed openings of the plurality of U-shaped tubes 12, then through tubes 12 to be introduced into the cold leg plenum 24. A circular tube sheet 28 is disposed at the bottom of cylindrical portion 14. Tube sheet 28 is divided into a first semi-circularly shaped half 30 and a second semi-circularly-shaped half 32 by divider plate 20. As shown in FIG. 1, the water entry openings of the tubes 12 are supported within openings in first tube sheet half 30, while the water exit openings of the tubes 12 are supported within openings in second tube sheet half 32.
Corrosion of the U-shaped tubes 12 is a concern in certain steam generators. Corrosion is usually the result of chemical attack and can occur on the outside surface, which is exposed to boiler steam and on the inside surface, which is exposed to water. Water chemistry treatment has been used in the past to protect the U-shaped tubes 12, as well as other system components, from corrosion. However, chemical treatment will not be successful unless all factors contributing to corrosion are detected and appreciated. Once corrosion has occurred, maintenance of the tubes 12 is typically effected by removing from service a defective tube by "plugging" each end. "Plugging" is carried out by entering a first portion of the channel head 16 to seal first one end of a defective tube 12 and then entering the second portion of the channel head 12 to seal the other end of the tube 12. This procedure is complicated and time consuming.